The present invention relates to variable bandwidth filters, more particularly to those having means for maintaining the output amplitude constant as the bandwidth is varied.
There are many measurement conditions or devices for which it is desirable to have a variable bandwidth filter that produces an output signal having constant amplitude regardless of the selected bandwidth. A spectrum analyzer is one such device. The bandwidth of any bandpass filter can be varied by varying its loading, or Q, wherein Q=.omega.L/R and BW.varies.1/Q, however, unless compensated for, this also varies the amplitude of the output signal of the filter. Some prior art devices have the bandwidth and gain controls ganged, but this is undesirable since a match between two separate circuits is hard to achieve.
Another approach was presented in U.S. Pat. No. 3,348,161 to Ranky. The configuration of the filter disclosed therein is shown in FIG. 1 in block diagram form. This design includes a cascaded arrangement of an input amplifier, a filter, a low input impedance amplifier with a bandwidth and gain control resistor in series with one of its input terminals and a shunt tank coupled to the other of its input terminals wherein the tank has a resonate frequency which is substantially the same as the center frequency of the filter. The output signal of the low input impedance amplifier and the signal at the junction of the tank and low input impedance amplifier is applied to a summer with the sum of the signals applied to the input terminal of a final high input impedance amplifier having both high impedance and low impedance output terminals. The operation of this circuit will be discussed in more detail below. This circuit is undesirable since its operation is adversely influenced by the noise voltage which is developed across R.sub.BW, the variable resistor for controlling the gain of the low input impedance amplifier and the overall bandwidth of the filter.
It would be desirable to have a variable bandwidth filter which utilizes a single control element which, when varied, causes the variation of the bandwidth of the filter while maintaining the output amplitude constant without the undesirable effects introduced by the noise voltage of the prior art circuit. The amplitude compensated variable bandwidth filter of the present invention achieves each of the above-mentioned results.